The flag of the USA was one of the earliest flags to represent political subdivisions on a national flag. Indeed, the US flag was arguably the very first "national flag" as it represented a nation rather than a monarchy (the flag of Denmark is older, but the Danebrog was the flag of the Kingdom of Denmark, not the "nation" as we know it today.) The Union Flag of the United Kingdom represented England and Scotland by combining two distinct crosses. And that flag, commonly called the "Union Jack" is only the "national flag" of the United Kingdom by tradition and by permission, not by law.

For more info on the US flag's design elements, see Peter Ansoff's link in his March 27 post.